Methods and semiconductor structures for latch-up suppression using a conductive region

ABSTRACT

Semiconductor structures and methods for suppressing latch-up in bulk CMOS devices. The semiconductor structure comprises first and second adjacent doped wells formed in the semiconductor material of a substrate. A trench, which includes a base and first sidewalls between the base and the top surface, is defined in the substrate between the first and second doped wells. The trench is partially filled with a conductor material that is electrically coupled with the first and second doped wells. Highly-doped conductive regions may be provided in the semiconductor material bordering the trench at a location adjacent to the conductive material in the trench.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/340,752,filed Jan. 26, 2006, which is hereby incorporated by reference herein inits entirety.

FIELD OF THE INVENTION

The invention relates generally to semiconductor structures and methodsand, in particular, to semiconductor structures for reducing orsuppressing latch-up in bulk complementary metal-oxide-semiconductordevice structures and methods for fabricating these semiconductorstructures.

BACKGROUND OF THE INVENTION

Complementary metal-oxide-semiconductor (CMOS) technologies integrate P-and N-channel field effect transistors (FETs) to form an integratedcircuit on a single semiconductor substrate. Latch-up, which isprecipitated by unwanted transistor action of parasitic bipolartransistors inherently present in bulk CMOS devices, may be asignificant issue for bulk CMOS technologies. The unwanted parasitictransistor action, which has various triggers, may cause failure of bulkCMOS devices. For space-based applications, latch-up may be induced bythe impingement of high energy ionizing radiation and particles (e.g.,cosmic rays, neutrons, protons, alpha particles). Because the integratedcircuit cannot be easily replaced in space flight systems, the chipfailure may prove catastrophic. Hence, designing bulk CMOS devices witha high tolerance to latch-up is an important consideration for circuitoperation in the natural space radiation environment, as well asmilitary systems and high reliability commercial applications.

Bulk CMOS device designs may be adjusted to increase latch-up immunity.For example, latch-up immunity may be increased in 0.25 micron devicetechnologies by building bulk CMOS devices on epitaxial substrates(e.g., a p-type epitaxial layer on a highly-doped p-type substratewafer). Highly-doped substrate wafers provide excellent current sinksfor currents that, if unabated, may initiate latch-up. However,epitaxial substrates are expensive to produce and may increase thedesign complexity of several critical circuits, such as electrostaticdischarge (ESD) protective devices.

Guard ring diffusions represent another conventional approach forsuppressing latch-up. However, guard ring diffusions are costly becausethey occupy a significant amount of active area silicon real estate. Inaddition, although guard ring diffusions collect a majority of theminority carriers in the substrate, a significant fraction may escapecollection underneath the guard ring diffusion.

Semiconductor-on-insulator (SOI) substrates are recognized by thesemiconductor community as generally free of latch-up. However, CMOSdevices are expensive to fabricate on an SOI substrate, as compared to abulk substrate. Furthermore, SOI substrates suffer from various otherradiation-induced failure mechanisms aside from latch-up. Anotherdisadvantage is that SOI devices do not generally come with a suite ofASIC books that would enable simple assembly of low-cost designs.

Conventional CMOS devices are susceptible to latch-up generally becauseof the close proximity of N-channel and P-channel devices. For example,a typical CMOS device fabricated on a p-type substrate includes aP-channel transistor fabricated in an N-well and an N-channel transistorfabricated in a P-well. The opposite conductivity N- and P-wells areseparated by only a short distance and adjoin across a well junction.This densely-packed bulk CMOS structure inherently forms a parasiticlateral bipolar (PNP) structure and parasitic vertical bipolar (NPN)structure. Latch-up may occur due to regenerative feedback between thesePNP and NPN structures.

With reference to FIG. 1, a portion of a standard triple-well bulk CMOSstructure 30 (i.e., CMOS inverter) includes a P-channel transistor 10formed in an N-well 12 of a substrate 11, an N-channel transistor 14formed in a P-well 16 of the substrate 11 that overlies a buried N-band18, and a shallow trench isolation (STI) region 20 separating the N-well12 from the P-well 16. Other STI regions 21 are distributed across thesubstrate 11. The N-channel transistor 14 includes n-type diffusionsrepresenting a source 24 and a drain 25. The P-channel transistor 10 hasp-type diffusions representing a source 27 and a drain 28. The N-well 12is electrically coupled by a contact 19 with the standard power supplyvoltage (V_(dd)) and the P-well 16 is electrically coupled by a contact17 to the substrate ground potential. The input of the CMOS structure 30is connected to a gate 13 of the P-channel transistor 10 and to a gate15 of the N-channel transistor 14. The output of CMOS structure 30 isconnected to the drain 28 of the P-channel transistor 10 and the drain25 of the N-channel transistor 14. The source 27 of the P-channeltransistor 10 is connected to V_(dd) and the source 24 of the N-channeltransistor 14 is coupled to ground. Guard ring diffusions 34, 36encircle the CMOS structure 30.

The n-type diffusions constituting the source 24 and drain 25 of theN-channel transistor 14, the isolated P-well 16, and the underlyingN-band 18 constitute the emitter, base, and collector, respectively, ofa vertical parasitic NPN structure 22. The p-type diffusionsconstituting the source 27 and drain 28 of the P-channel transistor 10,the N-well 12, and the isolated P-well 16 constitute the emitter, base,and collector, respectively, of a lateral parasitic PNP structure 26.Because the N-band 18 constituting the collector of the NPN structure 22and the N-well 12 constituting the base of the PNP structure 26 areshared and the P-well 16 constitutes the base of the NPN structure 22and also the collector of the PNP structure 26, the parasitic NPN andPNP structures 22, 26 are wired to result in a positive feedbackconfiguration.

A disturbance, such as impinging ionizing radiation, a voltage overshooton the source 27 of the P-channel transistor 10, or a voltage undershooton the source 24 of the N-channel transistor 14, may result in the onsetof regenerative action. This results in negative differential resistancebehavior and, eventually, latch-up of the bulk CMOS structure 30. Inlatch-up, an extremely low-impedance path is formed between emitters ofthe vertical parasitic NPN structure 22 and the lateral parasitic PNPstructure 26, as a result of the bipolar bases being flooded withcarriers. The low-impedance state may precipitate catastrophic failureof that portion of the integrated circuit. The latched state may only beexited by removal of, or drastic lowering of, the power supply voltagebelow the holding voltage. Unfortunately, irreversible damage to theintegrated circuit may occur almost instantaneously with the onset ofthe disturbance so that any reaction to exit the latched state isbelated.

What is needed, therefore, is a semiconductor structure and fabricationmethod for modifying standard bulk CMOS device designs that suppresseslatch-up, while being cost effective to integrate into the process flow,and that overcomes the disadvantages of conventional bulk CMOSsemiconductor structures and methods of manufacturing such bulk CMOSsemiconductor structures.

SUMMARY OF THE INVENTION

The present invention is generally directed to semiconductor structuresand methods that improve latch-up immunity or suppression in standardbulk CMOS device designs, while retaining cost effectiveness forintegration into the process flow forming the P-channel and N-channelfield effect transistors characteristic of bulk CMOS devices. Inaccordance with an embodiment of the present invention, a semiconductorstructure comprises a substrate of a semiconductor material and firstand second doped wells formed in the semiconductor material of thesubstrate. The second doped well is disposed adjacent to the first dopedwell. A trench, which includes a base and first sidewalls between thebase and the top surface, is defined in the substrate between the firstand second doped wells. The trench is partially filled with a conductormaterial that electrically couples the first and second doped wells.

In accordance with another embodiment of the present invention, a methodis provided for fabricating a semiconductor structure in a substrate ofsemiconductor material having a first doped well and a second doped welladjacent to the first doped well. The method comprises forming a trenchin the semiconductor material with first sidewalls disposed between abase of the trench and a top surface of the substrate and partiallyfilling the trench with a first conductive region to establish aconductive path between the first and second doped wells.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given above and thedetailed description of the embodiments given below, serve to explainthe principles of the invention.

FIG. 1 is a diagrammatic view of a portion of a substrate with a bulkCMOS device constructed in accordance with the prior art.

FIGS. 2-8 are diagrammatic views of a portion of a substrate at variousfabrication stages of a processing method in accordance with anembodiment of the present invention.

FIG. 8A is a top view of the substrate portion at the fabrication stageof FIG. 8 with the P-channel and N-channel transistors omitted forclarity.

FIGS. 9-14 are diagrammatic views of a portion of a substrate at variousfabrication stages of a processing method in accordance with analternative embodiment of the present invention.

FIGS. 15-20 are diagrammatic views of a portion of a substrate atvarious fabrication stages of a processing method in accordance with analternative embodiment of the present invention.

FIGS. 21-25 are diagrammatic views of a portion of a substrate atvarious fabrication stages of a processing method in accordance with analternative embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides an isolation region that limits theeffect of the vertical parasitic NPN structure and the lateral parasiticPNP structure responsible for latch-up in triple-well bulk CMOS devices.The invention is advantageously implemented in the context of bulk CMOSdevices where pairs of N-channel and P-channel field effect transistorsare formed adjacent to each other in a P-well within an N-band and anN-well, respectively, and the P-well is isolated from the N-well by ashallow trench isolation (STI) region. Specifically, the latchupimmunity of a standard bulk CMOS triple well structure is improved bymodifying the STI region with a conductive region or bridge thatoperates to significantly reduce the susceptibility to latch-up. Thepresent invention will now be described in greater detail by referringto the drawings that accompany the present application.

With reference to FIG. 2, a bulk substrate 40 of a monocrystallinesemiconductor material is obtained from, for example, a commercialsupplier. Substrate 40 may include a low-defect epitaxial layer fordevice fabrication that is grown by an epitaxial growth process, such aschemical vapor deposition (CVD) using a silicon source gas (e.g.,silane). Substrate 40 may be a single crystal silicon wafer containing arelatively light concentration of a dopant providing p-typeconductivity. For example, substrate 40 may be lightly doped with 5×10¹⁵cm⁻³ to 1×10¹⁷ cm⁻³ of a p-type dopant, such as boron, by in situ dopingduring deposition of the epitaxial layer.

A pad structure 42 is formed on a top surface 41 of the substrate 40.The pad structure 42 includes a first pad layer 44 separated from thesubstrate 40 by a thinner second pad layer 46. The material(s) formingpad layers 44, 46 advantageously etch selectively to the constituentsemiconductor material of substrate 40. The first pad layer 44 may be aconformal layer of nitride (Si₃N₄) formed by a thermal CVD process likelow pressure chemical vapor deposition (LPCVD) or a plasma-assisted CVDprocess. The second pad layer 46, which may be silicon oxide (SiO₂)grown by exposing substrate 40 to either a dry oxygen ambient or steamin a heated environment or deposited by a thermal CVD process, mayoperate as a buffer layer to prevent any stresses in the materialconstituting the first pad layer 44 from causing dislocations in thesemiconductor material of substrate 40.

A plurality of shallow trenches, of which a representative shallowtrench 48 is shown in FIG. 2, are formed in the semiconductor materialof substrate 40 by a conventional lithography and subtractive etchingprocess that utilizes a shallow trench pattern imparted in the padstructure 42 or, optionally, in a patterned hard mask (not shown) formedon pad structure 42. The shallow trench pattern may be created in padstructure 42 by applying a photoresist (not shown) on pad layer 44,exposing the photoresist to a pattern of radiation to create a latentshallow trench pattern in the photoresist, and developing the latentshallow trench pattern in the exposed photoresist. An anisotropic dryetching process, such as reactive-ion etching (RIE) or plasma etching,may then be used to transfer the trench pattern from the patternedresist into the pad layers 44, 46. The etching process, which may beconducted in a single etching step or multiple etching steps withdifferent etch chemistries, removes portions of the pad structure 42visible through the shallow trench pattern in the patterned resist andstops vertically on the substrate 40. After etching is concluded,residual resist is stripped from the pad structure 42 by, for example,plasma ashing or a chemical stripper.

The shallow trench pattern is then transferred from the patterned padlayer 44 into the underlying substrate 40 with an anisotropic dryetching process. The anisotropic dry etching process may be constitutedby, for example, RIE, ion beam etching, or plasma etching using an etchchemistry (e.g., a standard silicon RIE process) that removes theconstituent semiconductor material of substrate 40 selective to thematerials constituting the pad layers 44, 46. The shallow trench 48defined in the semiconductor material of substrate 40 includes oppositesidewalls 50, 52, which are substantially mutually parallel and orientedsubstantially perpendicular to the top surface 41 of substrate 40, thatextend into the substrate 40 to a bottom surface or base 54. The shallowtrench 48 is registered or aligned with a corresponding opening 55 ofthe shallow trench pattern in the pad structure 42.

With reference to FIG. 3 in which like reference numerals refer to likefeatures in FIG. 2 and at a subsequent fabrication stage, spacers 56, 58are formed on the sidewalls 50, 52 of shallow trench 48 and thecorresponding aligned opening 55 in the pad structure 42. Spacers 56, 58may be defined from a conformal layer (not shown) of a dielectricmaterial, such as silicon oxide deposited by a CVD process, that isanisotropically etched using a reactive ion etch (RIE) or plasma etchingprocess to remove the material of the conformal layer primarily fromhorizontal surfaces selective to (i.e., with a significantly greateretch rate than) the constituent semiconductor material of substrate 40.The base 54 of shallow trench 48 is exposed after the spacers 56, 58 areformed. The semiconductor material of substrate 40 etches selectively tothe dielectric material forming the spacers 56, 58, which guides theselection of the dielectric material. A person having ordinary skillwill appreciate that additional shallow trenches (not shown), similar toshallow trench 48, defined in the substrate 40 will include spacers (notshown) similar to spacers 56, 58. A block mask (not shown) may be usedto cover certain shallow trenches (not shown) in the shallow trenchpattern during spacer formation.

With reference to FIG. 4 in which like reference numerals refer to likefeatures in FIG. 3 and at a subsequent fabrication stage, an anisotropicetching process is used to deepen shallow trench 48, and other shallowtrenches similar to trench 48, to define a pigtail or vertical extension60, also referred to as a deep trench. The vertical extension 60 has abase 62 and sidewalls 64, 66 disposed between base 62 and base 54. Thepad structure 42 and the spacers 56, 58 operate as an etch mask forsemiconductor material in the covered regions of substrate 40. Theabsolute depths to which the shallow trench 48 and vertical extension 60are etched may vary with particular device designs. A person havingordinary skill will appreciate that additional shallow trenches (notshown), similar to shallow trench 48, defined in the substrate 40 willinclude a vertical extension similar to vertical extension 60. Shallowtrenches (not shown) masked by the optional block mask are unaffected bythe anisotropic etching process forming vertical extension 60.

With reference to FIG. 5 in which like reference numerals refer to likefeatures in FIG. 4 and at a subsequent fabrication stage, a dielectricregion 68 is formed in the vertical extension 60 adjacent to base 62.The insulating or dielectric material forming the dielectric region 68may be a high-density plasma (HDP) oxide, which deposits predominantlyon horizontal surfaces. An extraneous dielectric layer 70 may be formedacross the pad structure 42. Any dielectric material deposited on thevertical surfaces of the spacers 56, 58 or on the sidewalls 64, 66 ofvertical extension 60 may be removed by a brief isotropic etchingprocess.

With reference to FIG. 6 in which like reference numerals refer to likefeatures in FIG. 5 and at a subsequent fabrication stage, a conductivebridge or region 72 is formed in the vertical extension 60 such that thedielectric region 68 separates the conductive region 72 from the base62. The conductive region 72 comprises a portion of a conformal layer ofan appropriate conductor or conductive material that is deposited in thevertical extension 60. For example, the conductor constituting theconductive region 72 may be polycrystalline silicon (polysilicon)deposited conformally by a CVD process and in situ doped during the CVDprocess with a dopant having, for example, an n-type conductivity. Thedopant in polysilicon may be, for example, arsenic or phosphorus.Although the invention is not so limited, persons having ordinary skillin the art can appreciate that arsenic may be advantageous for use indoping the conductive material of conductive region 72 because of therelatively low diffusion rate for arsenic in silicon. Other shallowtrenches (not shown) in the shallow trench pattern having verticalextensions similar to vertical extension 60 will likewise receive aconductive region similar to conductive region 72.

Portions of the conformal conductor layer overlying dielectric layer 70are removed by a planarization process, such as a chemical-mechanicalpolishing (CMP) process. The top surface of dielectric layer 70 operatesas a vertical polish stop for the planarization process. Afterplanarization, the depth of a top surface of conductive region 72 isrecessed vertically within the vertical extension 60 by an anisotropicdry etching process, such as an RIE or plasma etching process, thatremoves the constituent conductor of the conductive region 72 selectiveto the materials constituting the substrate 40 and dielectric layer 70.The top surface of the conductive region 72 is significantly recessedbelow the top surface 41 of substrate 40. The vertical extent of theconductive region 72 may vary according to the device design. A changein the vertical extent of the conductive region 72 results in amodification of the vertical extent of the conductive regions 74, 75(FIGS. 8, 8A).

With reference to FIG. 7 in which like reference numerals refer to likefeatures in FIG. 6 and at a subsequent fabrication stage, a dielectricregion 71 is formed by filling the open volume in the vertical extension60 above the conductive region 72 and the open volume in the shallowtrench 48 with an insulator or dielectric material, such as HDP oxide ortetraethylorthosilicate (TEOS). The dielectric material filling thepreviously open volumes in vertical extension 60 and shallow trench 48may originate from a deposited layer that is planarized to the topsurface of the pad structure 42 by, for example, a CMP process. A hightemperature process step may be used to densify the TEOS fill. The topsurface 41 of substrate 40 is exposed by removing pad structure 42 usinga suitable process.

The dielectric regions 68, 71 and the conductive region 72 collectivelyconstitute an isolation region 73. Other isolation regions (not shown),similar to isolation region 73, are formed concurrently with isolationregion 73 as the dielectric material of dielectric region 71 fills theother shallow trenches (not shown) in the shallow trench pattern and theassociated vertical extension, similar to vertical extension 60.

With reference to FIGS. 8 and 8A in which like reference numerals referto like features in FIG. 7 and at a subsequent fabrication stage,standard bulk CMOS processing follows, which includes formation of atriple-well structure consisting of an N-well 76, a P-well 78, and adeep buried N-well or N-band 80 in the substrate 40. The buried N-band80 supplies electrical isolation for the P-well 78. This triple-wellconstruction permits the optimization of bias potentials for both N- andP-wells 76, 78. The P-well 78 is arranged between the N-band 80 and thetop surface 41 of substrate 40.

The N-band 80, as well as other N-bands (not shown) dispersed across thesubstrate 40, are formed by patterning a mask layer (not shown), such asa photoresist, applied on the top surface 41 and implanting anappropriate n-conductivity type impurity into the substrate 40 in thisset of unmasked regions. The P-well 78, as well as other P-wells (notshown) dispersed across the substrate 40, are likewise formed bypatterning another mask layer (not shown) applied on top surface 41 andimplanting an appropriate p-conductivity type impurity into thesubstrate 40 in this set of unmasked regions. Typically, the P-well 78is formed by counterdoping the N-band 80 and has an oppositeconductivity type from the N-well 76 and N-band 80. The N-well 76, aswell as other N-wells (not shown) dispersed across the substrate 40, arelikewise formed by patterning another mask layer (not shown) applied onthe top surface 41 with techniques known in the art and implanting anappropriate n-conductivity type impurity into the substrate 40 inunmasked regions. Generally, the dopant concentration in the N-well 76ranges from about 5.0×10¹⁷ cm⁻³ to about 7.0×10¹⁸ cm⁻³, the dopantconcentration in the P-well 78 ranges from about 5.0×10¹⁷ cm⁻³ to about7.0×10¹⁸ cm⁻³, and the dopant concentration in the N-band 80 ranges fromabout 5.0×10¹⁷ cm⁻³ to about 7.0×10¹⁸ cm⁻³. A thermal anneal may berequired to electrically activate the implanted impurities operating asthe p-type and n-type dopants.

An N-channel transistor 82 is built using the P-well 78 and a P-channeltransistor 84 is built using the N-well 76 to define a bulk CMOS device.The N-channel transistor 82 includes n-type diffusions in thesemiconductor material of substrate 40 representing a source region 86and a drain region 88 that flank opposite sides of a channel region inthe semiconductor material of substrate 40, a gate electrode 90overlying the channel region, and a gate dielectric 92 electricallyisolating the gate electrode 90 from the substrate 40. The P-channeltransistor 84 includes p-type diffusions in the semiconductor materialof substrate 40 representing a source region 94 and a drain region 96that flank opposite sides of a channel region in the semiconductormaterial of substrate 40, a gate electrode 98 overlying the channelregion, and a gate dielectric 100 electrically isolating the gateelectrode 98 from the substrate 40. Other structures, such as sidewallspacers (not shown), may be included in the construction of theN-channel transistor 82 and the P-channel transistor 84.

The conductor used to form the gate electrodes 90, 98 may be, forexample, polysilicon, silicide, metal, or any other appropriate materialdeposited by a CVD process, etc. The source and drain regions 86, 88 andthe source and drain regions 94, 96 may be formed in the semiconductormaterial of substrate 40 by ion implantation of a suitable dopantspecies having an appropriate conductivity type. The gate dielectrics92, 100 may comprise any suitable dielectric or insulating material likesilicon dioxide, silicon oxynitride, a high-k dielectric, orcombinations of these dielectrics. The dielectric material constitutingdielectrics 92, 100 may be between about 1 nm and about 10 nm thick, andmay be formed by thermal reaction of the semiconductor material of thesubstrate 40 with a reactant, a CVD process, a physical vapor deposition(PVD) technique, or a combination thereof.

Processing continues to complete the semiconductor structure, includingforming electrical contacts to the gate electrodes 90, 98, source region86, drain region 88, source region 94, and drain region 96. The contactsmay be formed using any suitable technique, such as a damascene processin which an insulator is deposited and patterned to open vias, and thenthe vias are filled with a suitable conductive material, as understoodby a person having ordinary skill in the art. The N-channel andP-channel transistors 82, 84 are coupled using the contacts with otherdevices on substrate 40 and peripheral devices with a multilevelinterconnect structure consisting of conductive wiring and interleveldielectrics (not shown). The N-well 76 is electrically coupled with thestandard power supply voltage (V_(dd)) and the P-well 78 is electricallycoupled with the substrate ground potential.

During high temperature process steps associated with the formation ofthe semiconductor structure or during a dedicated thermal anneal,conductive regions 74, 75 are formed in the semiconductor material ofthe substrate 40 adjacent to the conductive region 72. The conductiveregions 74, 75, which are formed when the mobile dopant in theconductive region 72 thermally outdiffuses into the semiconductormaterial of the substrate 40, flank the conductive region 72. Theconductive regions 74, 75 possess a higher dopant concentration thaneither the N-well 76 or the N-band 80. Conductive regions (not shown)similar to conductive regions 74, 75 will form flanking other shallowtrenches (not shown) having a conductive region similar to conductiveregion 72.

In accordance with the principles of the invention, the conductiveregion 72 bridging vertical extension 60 and the conductive regions 74,75 in the semiconductor material of substrate 40 flanking conductiveregion 72 cooperate for bridging the N-well 76 on one side of theisolation region 73 and the N-band 80 on the opposite side of theisolation region 73. Because the base 62 of the vertical extension 60 isdeeper than the deepest boundary of the N-well 76 or N-band 80, holesemitted from the source region 94 of the P-channel transistor 84 aredirected to the conductive region 72 in the vertical extension 60 andthe conductive regions 74, 75 in the semiconductor material of substrate40 flanking conductive region 72. The holes rapidly recombine withelectrons in the conductive region 72, which reduces or eliminates thegain of the lateral parasitic PNP structure 26 (FIG. 1) because anegligible hole current is collected by the P-well 78 and, thereby,suppresses latch-up.

With reference to FIG. 9 in which like reference numerals refer to likefeatures in FIG. 2 and in accordance with an alternative embodiment ofthe present invention, the vertical extension 60 and conductive region72 may be formed before the shallow trench 48 is formed and filled withdielectric material to define isolation region 73. To that end, a deeptrench 104 is formed in the semiconductor material of substrate 40 usingan anisotropic etching process, as described above with regard to theformation of vertical extension 60 in FIG. 4. The anisotropic etchingprocess transfers a deep trench pattern including deep trench 104 fromthe patterned pad structure 42 to the semiconductor material of thesubstrate 40. The deep trench pattern includes additional deep trenches(not shown) similar to deep trench 104 and formed at locationsdistributed across the surface 41 of substrate 40. The patterned padstructure 42 operates as an etch mask for semiconductor material in thecovered regions of substrate 40. The absolute depth to which the deeptrench 104 is etched may vary contingent upon the particular devicedesign. The lower portion of the deep trench 104 constitutes thevertical extension 60.

With reference to FIG. 10 in which like reference numerals refer to likefeatures in FIG. 9 and at a subsequent fabrication stage, the dielectricregion 68 is formed in the vertical extension 60 adjacent to base 62. Asexplained above with regard to FIG. 5, the dielectric region 68 isformed by depositing HDP oxide or TEOS in the deep trench 104,planarizing the deposited layer, and recessing the residual planarizeddielectric material in the deep trench 104 relative to the top surface41 of substrate 40.

With reference to FIG. 11 in which like reference numerals refer to likefeatures in FIG. 10 and at a subsequent fabrication stage, a conductiveplug 106 is formed in the deep trench 104 above the dielectric region68. The conductive plug 106 comprises a portion of a conformal layer(not shown) constituted by an appropriate conductor or conductivematerial. The conformal layer may be polysilicon deposited conformallyby a CVD process and in situ doped during the CVD process with a dopanthaving, for example, an n-type conductivity (e.g., arsenic orphosphorus). Extraneous portions of the conformal layer overlying padstructure 42 are removed by a planarization process, such as achemical-mechanical polishing (CMP) process. The top surface of padlayer 44 operates as a vertical polish stop for the planarizationprocess.

With reference to FIG. 12 in which like reference numerals refer to likefeatures in FIG. 11 and at a subsequent fabrication stage, the shallowtrench pattern, which includes shallow trench 48, is formed in thesemiconductor material of substrate 40 by a conventional lithography andsubtractive etching process, as described above with regard to FIG. 2.Shallow trench 48 is formed in a registered and aligned relationshipwith deep trench 104 and, hence, completes the formation of verticalextension 60 by shortening the vertical extent of deep trench 104. Thetrench formation process also removes portions of the conductive plug106 to create the conductive region 72. The etching process may beconducted in a single etching step or multiple etching steps withdifferent etch chemistries for removing the diverse materials ofsubstrate 40 and conductive plug 106.

With reference to FIG. 13 in which like reference numerals refer to likefeatures in FIG. 12 and at a subsequent fabrication stage, the isolationregion 73 and other isolation regions (not shown) are formed by fillingthe shallow trench pattern, which includes shallow trench 48, with thedielectric material of dielectric region 71, as described above withregard to FIG. 7. Because the conductive region 72 extends vertically tothe horizontal level of the base 54 of shallow trench 48, the dielectricregion 71 fails to extend vertically into vertical extension 60,although the invention is not so limited.

With reference to FIG. 14 in which like reference numerals refer to likefeatures in FIG. 13 and at a subsequent fabrication stage, standard bulkCMOS processing follows, as described above with regard to FIGS. 8 and8A, that fabricates N-well 76, P-well 78, N-band 80, N-channeltransistor 82, and the P-channel transistor 84. The conductive regions74, 75 are formed in the semiconductor material of substrate 40 in aflanking relationship with conductive region 72 as a consequence of thebulk CMOS processing or a separate thermal annealing step. Theconductive region 72, which bridges vertical extension 60, and theconductive regions 74, 75 cooperate for electrically coupling the N-well76 on one side of the isolation region 73 and the N-band 80 on the otherside of the isolation region 73. Because the base 62 of the verticalextension 60 is deeper than the N-well 76 or N-band 80, holes emittedfrom the source region 94 of the P-channel transistor 84 are directed tothe conductive region 72 in the vertical extension 60 and conductiveregions 74, 75 in the semiconductor material of substrate 40 flankingconductive region 72.

With reference to FIG. 15 in which like reference numerals refer to likefeatures in FIG. 9 and in accordance with an alternative embodiment ofthe present invention, the deep trench 104 is formed, as described withregard to FIG. 9, and lined with a liner 108 of an insulating material.The liner 108 may be silicon oxide deposited by a CVD process. The liner108, as explained below, cooperates with the dielectric region 68 andconductive region 72 of isolation region 73 in a manner analogous todielectric region 68 (FIG. 5) and may be advantageous if the deep trench104 has a high aspect ratio.

With reference to FIG. 16 in which like reference numerals refer to likefeatures in FIG. 17 and at a subsequent fabrication stage, a temporaryplug 110 of an organic material is formed near the base 62 of thevertical extension 60. The temporary plug 110 may be a resist or anotherorganic planarizing material that is deposited inside the deep trench104 and etched back to reduce the vertical extent. Portions of the liner108 not masked by the temporary plug 110 are removed from deep trench104 by an isotropic etching process that removes the liner 108 selectiveto the semiconductor material of substrate 40.

With reference to FIG. 17 in which like reference numerals refer to likefeatures in FIG. 16 and at a subsequent fabrication stage, the temporaryplug 110 of organic material is removed by a suitable process and aconductive plug 112, similar to conductive plug 106, is formed in thedeep trench 104, as described above with regard to FIG. 11. Conductiveplug 112 is used in a subsequent fabrication stage to form theconductive region 72 (FIG. 18).

With reference to FIG. 18 in which like reference numerals refer to likefeatures in FIG. 17 and at a subsequent fabrication stage, the shallowtrench pattern, which includes shallow trench 48, is formed in thesemiconductor material of substrate 40 by a conventional lithography andsubtractive etching process as described above with regard to FIG. 2.The shallow trench 48 is formed in a registered and aligned relationshipwith deep trench 104 and, hence, completes the formation of verticalextension 60. The trench formation process removes portions of theconductive plug 112 to form conductive region 72. The etching processmay be conducted in a single etching step or multiple etching steps withdifferent etch chemistries for removing the diverse materials ofsubstrate 40 and conductive plug 112. The liner 108 operates as anelectrical insulator between the conductive region 72 and the nearbysemiconductor material of the substrate 40 near the base 62 of thevertical extension 60.

With reference to FIG. 19 in which like reference numerals refer to likefeatures in FIG. 18 and at a subsequent fabrication stage, the isolationregion 73 and other isolation regions (not shown) are formed by fillingthe shallow trench pattern, which includes shallow trench 48, with thedielectric material of dielectric region 71, as described above withregard to FIG. 7. Because the conductive region 72 extends vertically tothe horizontal level of the base 54 of shallow trench 48, the dielectricregion 71 fails to extend vertically into vertical extension 60,although the invention is not so limited.

With reference to FIG. 20 in which like reference numerals refer to likefeatures in FIG. 19 and at a subsequent fabrication stage, standard bulkCMOS processing follows, as described above with regard to FIGS. 8 and8A, that fabricates N-well 76, P-well 78, N-band 80, N-channeltransistor 82, and the P-channel transistor 84. The conductive regions74, 75 form in the semiconductor material of substrate 40 in a flankingrelationship with conductive region 72 as a consequence of the bulk CMOSprocessing or a separate thermal annealing step. The conductive region72, which bridges vertical extension 60, and the conductive regions 74,75 cooperate for electrically coupling the N-well 76 on one side of theisolation region 73 and the N-band 80 on the other side of the isolationregion 73. Because the base 62 of the vertical extension 60 is deeperthan the N-well 76 or N-band 80, holes emitted from the source region 94of the P-channel transistor 84 are directed to the conductive region 72in the vertical extension 60 and conductive regions 74, 75 in thesemiconductor material of substrate 40 flanking conductive region 72.

With reference to FIG. 21 in which like reference numerals refer to likefeatures in FIG. 16 and in accordance with an alternative embodiment ofthe present invention, plug 110 (FIG. 16) is removed by a suitableprocess and a conductive plug 114, similar to conductive plugs 106 and112, is formed in the deep trench 104, as described above with regard toFIGS. 11 and 16, respectively. However, the conductive plug 114partially fills the deep trench 104. A conductive plug 116 of adifferent conducting material is formed in the upper portion of deeptrench 104. Preferably, the conductive material constituting plug 114 issilicon and the conductive material constituting plug 116 is a metal,such as tungsten, that forms a silicide with silicon when subjected toappropriate conditions.

With reference to FIG. 22 in which like reference numerals refer to likefeatures in FIG. 21 and at a subsequent fabrication stage, theconducting materials in plugs 114, 116 are combined to form a fullysilicided region 118. The ratio of the conducting materials in plugs114, 116 determines whether region 118 is fully silicided, as depictedin FIG. 21, or partially silicided. In an alternative embodiment of thepresent invention, the silicided region 118 may be deposited directly bya suitable deposition process without depositing the individual plugs114, 116 and promoting formation of silicided region 118 from the plugs114, 116.

With reference to FIG. 23 in which like reference numerals refer to likefeatures in FIG. 22 and at a subsequent fabrication stage, the shallowtrench pattern, which includes shallow trench 48, is formed in thesemiconductor material of substrate 40 by a conventional lithography andsubtractive etching process, as described above with regard to FIG. 2.The shallow trench 48 is formed in a registered and aligned relationshipwith deep trench 104 and, hence, completes the formation of verticalextension 60. The trench formation process removes portions of silicidedregion 118 to form conductive region 72. The etching process may beconducted in a single etching step or multiple etching steps withdifferent etch chemistries for removing the diverse materials ofsubstrate 40 and region 118. For example, a first etching step with anetch chemistry that selectively removes the metal silicide may be usedto recess the silicided region 118 relative to the surface 41 ofsubstrate 40 to substantially form conductive region 72, followed by asecond etching step with a different etching chemistry that selectivelyremoves the semiconductor material of substrate 40 to form the trench48.

With reference to FIG. 24 in which like reference numerals refer to likefeatures in FIG. 23 and at a subsequent fabrication stage, the isolationregion 73 and other isolation regions (not shown) are formed by fillingthe shallow trench pattern, which includes shallow trench 48, with thedielectric material of dielectric region 71, as described above withregard to FIG. 7. Because the conductive region 72 extends vertically tothe horizontal level of the base 54 of shallow trench 48, the dielectricregion 71 fails to extend vertically into vertical extension 60,although the invention is not so limited.

With reference to FIG. 25 in which like reference numerals refer to likefeatures in FIG. 24 and at a subsequent fabrication stage, standard bulkCMOS processing follows, as described above with regard to FIGS. 8 and8A, that fabricates N-well 76, P-well 78, N-band 80, N-channeltransistor 82, and the P-channel transistor 84. The conductive regions74, 75 form in the semiconductor material of substrate 40 in a flankingrelationship with conductive region 72 as a consequence of the bulk CMOSprocessing or a separate thermal annealing step. The conductive region72, which bridges vertical extension 60, and the conductive regions 74,75 cooperate for electrically coupling the N-well 76 on one side of theisolation region 73 and the N-band 80 on the other side of the isolationregion 73. Because the base 62 of the vertical extension 60 is deeperthan the N-well 76 or N-band 80, holes emitted from the source region 94of the P-channel transistor 84 are directed to the conductive region 72in the vertical extension 60 and conductive regions 74, 75 in thesemiconductor material of substrate 40 flanking conductive region 72.

References herein to terms such as “vertical”, “horizontal”, etc. aremade by way of example, and not by way of limitation, to establish aframe of reference. The term “horizontal” as used herein is defined as aplane parallel to the top surface 41 of substrate 40, regardless of itsactual spatial orientation. The term “vertical” refers to a directionperpendicular to the horizontal, as just defined. Terms, such as “on”,“above”, “below”, “side” (as in “sidewall”), “higher”, “lower”, “over”,“beneath” and “under”, are defined with respect to the horizontal plane.It is understood that various other frames of reference may be employedfor describing the present invention without departing from the spiritand scope of the present invention.

The fabrication of the semiconductor structure herein has been describedby a specific order of fabrication stages and steps. However, it isunderstood that the order may differ from that described. For example,the order of two or more fabrication steps may be switched relative tothe order shown. Moreover, two or more fabrication steps may beconducted either concurrently or with partial concurrence. In addition,various fabrication steps may be omitted and other fabrication steps maybe added. It is understood that all such variations are within the scopeof the present invention. It is also understood that features of thepresent invention are not necessarily shown to scale in the drawings.

While the present invention has been illustrated by a description ofvarious embodiments and while these embodiments have been described inconsiderable detail, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. Thus, the invention in its broader aspects istherefore not limited to the specific details, representative apparatusand method, and illustrative example shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of applicants' general inventive concept.

1. A method of fabricating a semiconductor structure in a substrate, themethod comprising: forming a trench in the semiconductor material withfirst sidewalls disposed between a base of the trench and a top surfaceof the substrate; partially filling the trench with a conductivematerial; forming a first doped well of the first conductivity type inthe semiconductor material of the substrate; forming a second doped wellof the first conductivity type in the semiconductor material of thesubstrate such that the first doped well is separated from the seconddoped well by the first sidewalls and the base of the trench; anddiffusing a mobile dopant of the first conductivity type from theconductive material laterally outward into the substrate along the firstsidewalls of the trench to define a first conductive region in the firstdoped well and a second conductive region in the second doped well eachhaving a higher dopant concentration of the first conductivity type thanthe first and second doped wells.
 2. The method of claim 1 furthercomprising: forming a first dielectric region in the trench between theconductive material and the base; and forming a second dielectric regionin the trench between the conductive material and the top surface. 3.The method of claim 2 wherein forming the first dielectric regionfurther comprises: lining at least the base of the trench with adielectric material.
 4. The method of claim 2 further comprising: beforethe first sidewalls are formed, forming second sidewalls extending fromthe top surface of the substrate into the substrate.
 5. The method claim4 further comprising: before the first sidewalls are formed, formingspacers on the second sidewalls to provide an etch mask for use informing the first sidewalls.
 6. The method of claim 5 wherein formingthe second dielectric region in the trench between the first conductiveregion and the top surface further comprises: filling an open volumebetween the second sidewalls with a portion of the second dielectricregion.
 7. The method of claim 2 further comprising: after the firstsidewalls are formed, widening the trench between the first sidewallsand the top surface to define second sidewalls.
 8. The method of claim 7wherein forming the second dielectric region in the trench between thefirst conductive region and the top surface further comprises: fillingan open volume between the second sidewalls with a portion of the seconddielectric region.
 9. The method of claim 1 wherein partially fillingthe trench with the first conductive region further comprises: forming ametal silicide in the trench that operates as the conductive material.10. The method of claim 9 wherein forming the metal silicide furthercomprises: partially filling the trench with a metal; partially fillingthe trench with polysilicon; and heating the metal and the polysiliconto promote formation of the silicide.